GB2455847A - A current transformer using magnetic elements to improve the core balance - Google Patents

Abstract

A current transformer comprises a core 10 having an opening 14 through which a plurality of primary conductors L, N pass. A secondary winding 12 is wound on the core 10. The transformer includes a respective ferromagnetic body 18 immediately on each side of the core 10 which surround the primary conductors L, N where they enter and exit the opening 14. The primary conductors L, N may be solid bus-bars rigidly coupled together by mechanical couplings 16. The ferromagnetic bodies 18 absorb stray magnetic fields, possibly produced by fringing effects, and may be formed by washer type elements of varying size and/or thickness or by a U-shaped member which is arranged to embrace the core 10 with ferromagnetic legs at either side of the core 10. An insulating tube may surround the primary conductors L, N as they pass through the core 10. The current transformer may be used in a residual current device. The current transformer may be cheap and easy to manufacture and assemble and with improved core balance characteristics it may provide a compact current transformer which is able to handle large currents efficiently with reduced risk of nuisance tripping.

Description

1 2455847 A Current Transformer This invention relates to a current transformer for, e.g., a residual current device.

Residual current devices (RCDs) detect earth fault currents, which are also known as residual currents. The principle of operation of RCDs is described in US Patent No. 7068047. Additional information can be found in the article Demystifying RCDs" at www. rcd. ie.

Figure 1 is an example of a simple RCD. In Figure 1, an AC mains supply comprising mains Live and neutral, conductors L, N is fed to a load LD via two normally-closed contacts S. En route to the load the mains conductors L, N pass through the toroidal. ferromagnetic core 10 of a current transformer CT.

The output of the CT is fed to an integrated circuit IC which may be of type WAO5O supplied by Western Automation Research Development. The integrated circuit IC is supplied with current via a resistor Ri from a bridge rectifier Xl. A solenoid SOL is connected from the bridge rectifier �ve to the bridge rectifier common via a silicon controlled rectifier SCR1, which is normally held in its non-conducting state by the IC. Some of the ancillary electronic circuitry has been omitted for simplicity and because it does not pertain to the invention.

The load conductors L, N form the primary winding of the current transformer CT, and the current transformer has a secondary winding 12 wound on the circumference of the core 10. A residual or differential current flow in the primary winding, i.e. an imbalance in the currents flowing to and from the load through the core 10, wiLl generate an electromagnetic field which will induce a current flow in the secondary winding 12 which will be detected by the electronic circuit. If above a certain threshold, this differential current wilL cause the RCD to trip by opening the contacts S. It foLlows that a balanced or non-differential current fLow in the primary conductors should not induce a current into the secondary winding and therefore not cause the RCD to trip.

In an electrical circuit with a purely resistive load, the initial inrush current when the circuit is turned on will be substantially the same as the steady state current. However, in circuits with reactive loads, the initial inrush current can be substantially greater than the subsequent steady state current, and in many cases will be several times larger than the steady state current. In early designs of RCD this resuLted in problems of nuisance tripping because the CT would produce an output in response to the inrush current of sufficient magnitude to cause the RCD to trip. This was unacceptable to users, and designers of RCDs had to take various measures to overcome this problem.

The cause of this problem is that in the absence of any earth leakage current flow in the circuit, the current passing through the CT is assumed to be a bInced current but the CT still produces an output current. The CT wilt only produce an output if an electromagnetic field induces a current into the secondary winding 12 of the CT. The problem of nuisance tripping in response to a large but balanced load current is referred to as a core balance problem. SeveraL factors are believed to contribute to this problem, such as: Orientation of the mains conductors L, N within the opening through the core -conductors passing through the core opening at an angLe other than right angles can produce asymmetrical magnetic fields within the CT with the result that the CT is subjected to a net magnetic flux.

Positioning of the mains conductors within the core opening -if the two conductors are spaced non-symmetricalLy within the opening with respect to the secondary winding 12, an asymmetrical field can be produced within the CT which can induce a current into the winding 12.

Symmetry of the winding 12 -this winding can comprise numerous turns, and if these are spaced unevenly around the circumference of the core, a current can be induced into the winding due to magnetic imbalance within the core.

The diameter of the core -for a given magnitude of Load current, e.g. 100A, smatter cores tend to be more susceptibLe to problems of core balance and nuisance tripping at high load currents.

Figure 2 shows how the current transformer might be arranged to achieve near perfect symmetry. In this case, where the conductors L, N pass through the opening in the core 10 they comprise rigid busbars which are symmetrically located within the core opening 14. The busbars can be insulated with a suitable varnish or with a sleeve. Ideally flexible wires would be welded to the busbars to route the conductors to circuit breaker elements or supply/load terminals, etc. However, such an arrangement can rarely be achieved in practice, and Figure 3 represents a more realistic arrangement.

In most cases the load conductors in an RCD will comprise multi-strand or solid wires within an insulating cover or sleeve rather than rigid busbars, and it can be very difficult to pass these through the core opening 14 in perfect symmetry. Also, in practice, on exiting the core 10 these conductors tend to be routed in various directions to connection points, so it is common practice for the conductors to be bent and formed to accommodate this as shown in Figure 3, aLl of which compromise symmetry.

Increasing the core size can help to ameliorate core balance problems.

However, this can be a very expensive solution because the inherent core cost is high anyway due to its technicaL characteristics, and the cost wilt generally increase at Least in proportion to core size. But core size itself can be severely restricted due to space constraints, and in many cases the use of a larger core may not a viable option.

The foLlowing provides an example of the scale of the core balance problem.

1EC61008 requires the RCD to withstand without tripping a continuous Load current of six times the rated load current for a time exceeding the rated trip time of the RCD.

The core balance test on a 63A/3OmA RCD requires a test current of 378A.

The core balance test for a 100A/3OmA RCD requires a test current of 600A. In each case it only requires an effective 3OmA residual current to trip the RCD.

A 0.05% imbalance in the 378A Load current could result in an effective residual current of 18.9mA, which is unlikeLy to trip a 3OmA RCD. However, a 0.05% imbalance in a 600A load current could result in an effective residual rurrpnf nf flmA. whit-h would trin flmA PCfl. Thic nrnvidpc n indirtinn of r------***-* the scale of the problem that has to be addressed by the RCD designer.

It is an object of the invention to provide an improved design of current transformer for, e.g., a residual current device.

The present invention provides a current transformer comprising a core having an opening through it, a plurality of primary conductors passing through the opening, and a secondary winding wound on the core, the transformer further including a respective ferromagnetic body immediately on each side of the core surrounding the primary conductors where they enter and exit the opening.

Preferably the core is a ferromagnetic toroidat core. Preferably, too, the ferromagnetic bodies are opposite legs of a generaUy U-shaped ferromagnetic member embracing the core.

The current transformer may include an insulating tube passing through the core and surrounding the primary conductors.

The invention further provides a residual current device having a current transformer as specified above.

Embodiments of the invention wiLt now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 (previously described) is a simplified bLock diagram of a typical RCD.

Figure 2 (previously described) shows schematic side and end views of an ideal arrangement of mains conductors relative to a toroidal ferromagnetic rf -i.rrnl frricfru-mr Figure 3 (previously described) shows similar views of a more likely arrangement of the mains conductors in a practical situation.

Figures 4 to 6 are schematic side views illustrating a further factor that can contribute to core balance problems and nuisance tripping in an RCD, and a first embodiment of the invention which mitigates such problems.

Figure 7 is a schematic side view of a second embodiment of the invention.

In the drawings the same reference numerals are used for the same or equivalent components.

In the current transformer of Figure 4, two solid busbars L, N were rigidly coupled together by mechanical couplings 16 and passed through the core 10 as precisely as possibLe so as to minimise asymmetry. The output of the secondary winding 12 was connected to a 3OmA RCD. A balanced load current was passed through the conductors and increased to a level that caused the RCD to trip. It could reasonably be assumed that at this level a magnetic field of sufficient magnitude was inducing a current into the winding 12 so as to represent a residual current of 3OmA and cause the RCD to trip.

A respective annular ferromagnetic washer 18 was then placed immediately on each side of the core 10 surrounding the conductors L, N where they enter and exit the opening 14, Figure 5, to determine the effect of placing ferromagnetic material in this position.

When the above test was repeated it was found that it required a significantly hiohr Inicl rairrnf tn riicp th PCI) tn trin Thi indir-trI that additional factor, referred to herein as fringing, was contributing to the core balance problem. This effect is demonstrated in Figure 6. Due to the fringing 20 at the entrance and exit of the opening 14, stray magnetic fields can be induced into the winding 12 so as to cause nuisance tripping at high load currents. Placing the ferromagnetic washers 18 on each side of the core 10 has the effect of absorbing the stray fields that can be caused by fringing.

Increasing the thickness of the washers 18 provides an increasing level of immunity to fringing up to a certain point. By substantially reducing the adverse influence of fringing, the washers also provide a higher degree of tolerance with regard to symmetry of the conductors through the core 10 and the winding 12 on the core. They also provide the added advantage of facilitating the use of a smaller and less expensive core.

The washers 18 should be placed as close as possible to the sides of the core for maximum effectiveness, and they also need to be adequately secured.

Additionally, they should be insulated from the conductors L, N. ALL of these problems can add to manufacturing complexity and cost. A simple but effective solution to these requirements is shown in 7.

In the arrangement of Figure 7, a ferromagnetic shield comprises a single pressed part formed into a generally U-shaped member 22. The opposite Legs 24 of the shield are substantially flat and paralLel, and each has an aperture 26 approximateLy the same diameter as the diameter of the core opening 14.

The shield 22 is placed over the core 10 so as to intimateLy embrace the latter on opposite sides with the shield apertures 26 aligned with the core opening 14. The mains conductors L, N are passed through the aligned openings 14, 26 and retain the shieLd 22 in position. An insulating tube 28 surrounding the conductors L, N can be passed through the aligned openings 14, 26 to improve insulation and anchoring.

The arrangement of Figure 7 provides a very simple, low cost and highly effective solution to providing a current transformer with a magnetic shield to overcome problems of core balance and unwanted tripping of RCDs.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims (6)

1. Claims: 1. A current transformer comprising a core having an opening through it, a plurality of primary conductors passing through the opening, and a secondary winding wound on the core, the transformer further including a respective ferromagnetic body immediately on each side of the core surrounding the primary conductors where they enter and exit the opening.
2. 2. A current transformer as claimed in cLaim 1 wherein the core is a ferromagnetic toroidat core.
3. 3. A current transformer as claimed in claim 2 wherein the ferromagnetic bodies are opposite Legs of a generally U-shaped ferromagnetic member embracing the core.
4. 4. A current transformer as claimed in claim 1 wherein the current transformer includes an insulating tube passing through the core and surrounding the primary conductors.
5. 5. A residual current device including a current transformer as claimed in any preceding claim.
6. 6. A current transformer or residual current device substantially as described herein with reference to and as illustrated in the accompanying drawings.